1. Field of the Invention
This invention relates to a high bit-rate coupler between a PCM channel bidirectional multiplex and a packet switching unit. The PCM channels carry words, such as octets, having a predetermined number of bits that are multiplexed word by word. Generally speaking, certain PCM channels are in a circuit mode and others in a packet mode. The switching unit is a packet-circuit switching unit switching both circuit mode channels carrying, for example, sampled speech and packet mode channels each carrying frames of one or more logical channels.
2. Description of the Prior Art
As specified in international recommendations, the frames in each packet mode channel are separated from each other by flags that are words having a predetermined configuration, such as 01111110, and each frame in the multiplex contains a filling zero after five contiguous bits "1" such that the frame may be suitably detected.
A high bit-rate coupler is analogous to an input-output interface and processes the first level related to the frame level of a packet exchange procedure. At the receiving end, the coupler extracts the filling zeros from the frames received from the multiplex so that the switching unit can process the frames at the second level in the frame level; at the procedure packet level, the coupler controls a frame checking sequence, counts the octets and the filling zeros extracted from each frame and detects the flags and the start and end of each frame. Conversely, at the transmitting end, the coupler inserts filling zeros into the frames delivered from the switching unit, derives the frame checking sequence in each frame, transmits the octets supplied from the switching unit, produces the flags and aborts certain frames or disables certain channels.
To extract the filling zeros in order to restore the initial frames having a predetermined number of words, the number of zeros already extracted from the already-received words of the same frame must be known. Conversely, to insert the filling zeros into the initial frames, the number of zeros already inserted in the already-transmitted words of the same frame must be known.
In known high bit-rate couplers, the processing operation performed on the received words to extract zeros and on the transmitted words to insert zeros is carried out on a frame-by-frame basis. A necessary condition for the processing operations is that the whole received or transmitted frame must be stored, following detection of the opening or closing flags of the frame. After storing the frame, zero extraction or insertion takes place on a delayed basis. The major drawbacks of such conditions reside in the need for an extensive memory capacity compared to the number of multiplexed PCM channels and to the number of words contained in the frames. Additionally, given that frame reception or transmission is not synchronous, filling zero inserting and extracting means must be assigned to each PCM channel.
Other known "compound" couplers comprise elementary couplers each assigned to one PCM channel, thereby incurring a high number of elementary couplers and links between the elementary couplers and the switching unit.